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Re: [Xen-devel] ResettRe: [v5][PATCH 0/5] xen: add Intel IGD passthrough support

On Wed, Jul 02, 2014 at 04:50:15PM +0200, Paolo Bonzini wrote:
> Il 02/07/2014 16:00, Konrad Rzeszutek Wilk ha scritto:
> >With this long thread I lost a bit context about the challenges
> >that exists. But let me try summarizing it here - which will hopefully
> >get some consensus.
> >
> >1). Fix IGD hardware to not use Southbridge magic addresses.
> >    We can moan and moan but I doubt it is going to change.
> There are two problems:
> - Northbridge (i.e. MCH i.e. PCI host bridge) configuration space addresses

Right. So in  drivers/gpu/drm/i915/i915_dma.c:
1135 #define MCHBAR_I915 0x44                                                   
1136 #define MCHBAR_I965 0x48                     

1147         int reg = INTEL_INFO(dev)->gen >= 4 ? MCHBAR_I965 : MCHBAR_I915;   
1152         if (INTEL_INFO(dev)->gen >= 4)                                     
1153                 pci_read_config_dword(dev_priv->bridge_dev, reg + 4, 
1154         pci_read_config_dword(dev_priv->bridge_dev, reg, &temp_lo);        
1155         mchbar_addr = ((u64)temp_hi << 32) | temp_lo;                


1139 #define DEVEN_REG 0x54                                                     

1193         int mchbar_reg = INTEL_INFO(dev)->gen >= 4 ? MCHBAR_I965 : 
1202         if (IS_I915G(dev) || IS_I915GM(dev)) {                             
1203                 pci_read_config_dword(dev_priv->bridge_dev, DEVEN_REG, 
1204                 enabled = !!(temp & DEVEN_MCHBAR_EN);                      
1205         } else {                                                           
1206                 pci_read_config_dword(dev_priv->bridge_dev, mchbar_reg, 
1207                 enabled = temp & 1;                                        
1208         }                                                
> - Southbridge (i.e. PCH i.e. ISA bridge) vendor/device ID; some versions of
> the driver identify it by class, some versions identify it by slot (1f.0).

Right, So in  drivers/gpu/drm/i915/i915_drv.c the giant intel_detect_pch
which sets the pch_type based on :

 432                 if (pch->vendor == PCI_VENDOR_ID_INTEL) {                  
 433                         unsigned short id = pch->device & 
 434                         dev_priv->pch_id = id;                             
 436                         if (id == INTEL_PCH_IBX_DEVICE_ID_TYPE) { 

It checks for 0x3b00, 0x1c00, 0x1e00, 0x8c00 and 0x9c00.
> To solve the first, make a new machine type, PIIX4-based, and pass through
> the registers you need.  The patch must document _exactly_ why the registers
> are safe to pass.  If they are not reserved on PIIX4, the patch must
> document what the same offsets mean on PIIX4, and why it's sensible to
> assume that firmware for virtual machine will not read/write them.  Bonus
> point for also documenting the same for Q35.

OK. They look to be related to setting up an MBAR , but I don't understand
why it is needed. Hopefully some of the i915 folks CC-ed here can answer.

> Regarding the second, fixing IGD hardware to not rely on chipset magic is a
> no-go, I agree.  I disagree that it's a no-go to define a "backdoor" that
> lets a hypervisor pass the right information to the driver without hacking
> the chipset device model.
> The hardware folks would have to give us a place for a pair of registers
> (something like data/address), and a bit somewhere else that would be always
> 0 on hardware and always 1 if the hypervisor is implementing the pair of
> registers.  This is similar to CPUID, which has the HYPERVISOR bit +
> hypervisor-defined leaves at 0x40000000.
> The data/address pair could be in a BAR, in configuration space, in the low
> VGA ports at 0x3c0-0x3df, wherever.  The hypervisor bit can be in the same
> place or somewhere else---again, whatever is convenient for the hardware
> folks.  We just need *one bit* that is known-zero on all hardware, and 8
> bytes in a reserved area.  I don't think it's too hard to find this space,
> and I really, really would like Intel to follow up on a paravirtualized
> backdoor.
> That said, we have the problem of existing guests, so I agree something else
> is needed.
> >     a) Two bridges - one 'passthrough' and the legacy ISA bridge
> >        that QEMU emulates. Both Linux and Windows are OK with
> >        two bridges (even thought it is pretty weird).
> This is pretty much the only solution for existing Linux guests that look up
> the southbridge by class.

> The proposed solution here is to define a new "pci stub" device in QEMU that
> lets you define a do-nothing device with your desired vendor ID, device ID,
> class and optionally subsystem IDs.

> The new machine type (the one that instantiates the special
> IGD-passthrough-enabled northbridge) can then instantiate this stub device
> at 1f.0 with the desired vendor ID, device ID and class ID.

Which is kind of neat because you can use a different type of device ID with 
(say make it look like Ibex Peak) and pair it up with an IGD that is found
only on LynxPoint. Oh fun!
> If we cannot get the paravirtualized backdoor, it would also make sense to:
> - have drivers standardize on a single way to probe the southbridge
> - make this be neither by class (because the firmware wants to distinguish
> the actual ISA bridge from the stub, and it can do so by looking up the
> class), nor by slot (because this conflicts with the Q35 chipset model that
> has the southbridge at 1f.0).
> mst's proposal was to probe by subsystem id.  I'm not sure I understood the
> details exactly, but I trust him. :)  However, in case it wasn't clear I
> think a paravirtualized backdoor would still be better.

OK, like this:

diff --git a/drivers/gpu/drm/i915/i915_drv.c b/drivers/gpu/drm/i915/i915_drv.c
index 651e65e..03f2829 100644
--- a/drivers/gpu/drm/i915/i915_drv.c
+++ b/drivers/gpu/drm/i915/i915_drv.c
@@ -433,6 +433,8 @@ void intel_detect_pch(struct drm_device *dev)
                        unsigned short id = pch->device & 
                        dev_priv->pch_id = id;
+                       if (pch->subsystem_vendor == PCI_VENDOR_ID_XEN)
+                               id = pch->device & INTEL_PCH_DEVICE_ID_MASK;
                        if (id == INTEL_PCH_IBX_DEVICE_ID_TYPE) {
                                dev_priv->pch_type = PCH_IBX;
                                DRM_DEBUG_KMS("Found Ibex Peak PCH\n");
> >     b) One bridge - the one that QEMU emulates - and lets emulate
> >        more of the registers (by emulate - I mean for some get the
> >        data from the real hardware).
> >
> >           b1). We can't use the legacy because the registers are
> >                above 256 (is that correct? Did I miss something?)
> As I understand it, mst brought up Q35 because the northbridge configuration
> space layout might be more similar to what the driver expects than for
> PIIX4.  But I don't think anyone really said whether this is true or false.
> I think Q35 is absolutely not a requirement for IGD passthrough, especially
> until this statement is either proved or disproved.

OK, so lets drop that.
> >4). Code does a bit of sysfs that could use some refacturing with
> >    the KVM code.
> >    Problem: More time needed to do the code restructing.
> FWIW, I don't really care about code sharing with KVM.  That's a separate
> problem and it's not necessary to bring it up and make waters even more
> muddy.

OK, lets drop that for now.
> Paolo

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